Nylon resin materials are superior to metal materials to obtain light-weight products, and also to obtain various excellent properties such as vibration-damping properties, rigidity, heat resistance, oil resistance and others. For these reason, nylon resins are recently used as molding materials in producing automobile parts, especially such parts employed around engines as cylinder head covers and gearcases, for the purpose of reducing automobile weights and engine noises.
In general, resin materials are superior to metal materials in vibration-damping performance. However, from the viewpoint of the effect for reducing noises, the substitution of resinous parts for metallic parts results in increase of air-borne sounds due to the small specific gravities of the resinous parts, although structure-borne sounds, i.e., sounds resulting from the vibrations of the parts themselves are reduced. Therefore, in order to effectively reduce noises by replacing metallic parts with resinous ones, the resinous parts should be able to reduce structure-borne sounds to such an extent that the resulting increase of air-borne sounds is more than counterbalanced by the reduction of structure-borne sounds. In other words, resinous parts are required to have extremely high vibration-damping performance.
In this respect, molded automobile parts of the above-mentioned kinds manufactured from conventional nylon resin materials are insufficient in vibration-damping performance, or even if they show relatively good vibration-damping properties at normal temperatures, their vibration-damping performance becomes insufficient in the practical temperature range, i.e., at temperatures as high as about 80.degree. C. to 120.degree. C. Thus, conventional nylon resin materials have such a technical problem to be solved.
In order to improve such insufficient ability to damp vibrations under the high-temperature use conditions, there has been proposed a resin composition comprising a polyamide resin and a tackifier resin as described in JP-A-61-36357 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, there are problems to be solved concerning the strength and rigidity of molded parts prepared by these resin composition.
Plastic materials generally possess highly viscoelastic properties, and their vibration-damping performance is most remarkable around temperatures at which the loss factors (.eta.) of the mechanical dispersion mainly due to the glass transition of the polymer materials have maximal values. Such temperatures are usually 10.degree. to 30.degree. C. higher than the glass transition temperatures (Tg) measured by differential scanning calorimetry (DSC). For example, straight-chain aliphatic nylons such as nylon 6, nylon 66, nylon 12, nylon 11 and nylon 610 have the Tg of about 40.degree. to 60.degree. C., and their vibration-damping performance is maximal at about 60.degree. to 80.degree. C. Aromatic nylons, as another type of nylon resin, such as a crystalline m-xylylenediamine resin (MXD nylon) which has aromatic rings in its molecular chains, and a copolymer of one or more of diamines and one or more of dicarboxylic acids (e.g., terephthalic acid, isophthalic acid) which is a non-crystalline nylon, have the Tg of 120.degree. C. or higher, with their vibration-damping performance being maximal at temperatures as high as 130.degree. C. or higher.